Novel exoenzyme toxin of Aeromonas salmonicida, and uses therefor

ABSTRACT

A novel protein toxin named  Aeromonas salmonicida  exoenzyme T (AexT), which belongs to the family of ADP-ribosylating toxins, is disclosed as is a novel Calcium (or other cation concentration) dependent promoter of  A. salmonicida.  Also disclosed are diagnostic, preventative, and therapeutic techniques, including the preparation of traditional, recombinant, and improved bacterin vaccines based on AexT for inducing immunity against  A. salmonicida  infections.

FIELD OF THE INVENTION

This invention relates to bacterial toxins and to bacterial promoters,and in particular to a newly identified and characterized toxin andpromoter of Aeromonas salmonicida. The invention also encompassesmethods for the production or accumulation of the A. salmonicida toxin,as well as the diagnostic, therapeutic, and preventative use (includingin particular the preparation of traditional or recombinant protein orDNA vaccines) of the A. salmonicida toxin. Also encompassed are methodsfor improving A. salmonicida bacterin vaccines by controlling expressionof the toxin.

BACKGROUND OF THE INVENTION

The fish disease furunculosis derived its name from the characteristiclesions observed as furuncles formed on the surface of fish as a resultof infection with Aeromonas salmonicida. This pathogen causes mostsevere losses in production farms of salmon and trout, and leads to theuse of large amounts of antibiotics in closed and open waters fortherapy of furunculosis. In order to develop efficient strategies toprevent A. salmonicida outbreaks, it is essential to know the mainmechanisms of pathogenicity of A. salmonicida.

Several potential virulence factors of A. salmonicida have beendescribed thus far. They include the surface array-layer protein, thehemolysins ASH1, ASH3, ASH4, H-lysin, salmolysin, the serine proteaseAspA and the Glycerophospholipid:Cholesterol Acyltransferase (GCAT)complexed with lipopolysaccharide (LPS). While there are many reports onpotential virulence factors of A. salmonicida, in particular hemolysins,little is known about their activity and their role in pathogenesis.Many of them seem not to play a primary role in pathogenesis, sincedeletion mutants of GCAT and aspA genes showed neither of them to beessential for acute A. salmonicida-induced furunculosis. AspA however isessential for pro-GCAT processing in broth cultures and might also beinvolved in activation of other secreted enzymes or toxins.

Various attempts have been made to develop vaccines to prevent A.salmonicida infections mainly on the basis of killed cells (bacterins).Current vaccines achieve some level of protection. However, the natureof the antigens in efficient vaccines is not well defined. Significantdifferences of protein patterns are seen in cultures of A. salmonicidagrown in vivo by an intraperitoneal implant technique in rainbow troutcompared to cultures grown in vitro in culture medium. Such differencesare thought to be the reasons of variable efficacy of formerfurunculosis vaccines due to a lack of appropriate antigens in certainvaccine preparations.

Several pathogenic bacteria use ADP-ribosylation as a key mechanism tomodify properties of host cell proteins, thus to modulate their functionand induce disease. Hence ADP-ribosylation of eukaryotic regulatoryproteins is the underlying pathogenic mechanism of a heterogeneousfamily of bacterial protein toxins. ADP-ribosylating toxins are broadlydistributed among highly pathogenic bacteria and are the primary causeof various severe human diseases such as diphtheria, cholera andpertussis. Among them, the ADP-ribosyltransferase toxin called exoenzymeS (ExoS) of Pseudomonas aeruginosa is one of the most prominentrepresentatives. It is secreted via a type III-dependent secretionmechanism. Type III secretion systems generally have the potential torecognize receptors on target cells, induce biosynthesis of thecorresponding toxins, and finally inject these bacterial toxins directlyinto the host cells without secretion to the medium. Recently, it wasshown that ExoS is a bifunctional toxin containing an N-terminal partresembling the Yersiniae YopE toxin which catalyses rho-dependent actindepolymerisation, and a C-terminal ADP-ribosylating domain. Unique tomost bacterial toxins, the ADP-ribosylating toxin ExoS does not have arigid target protein specificity and ribosylates a number of targetproteins including IgG3, apolipoprotein A-I, vimentin and severalmembers of the Ras superfamily. Intracellular expression of theamino-terminal domain of ExoS elicits the disruption of actin, whileexpression of the carboxyl-terminal domain of ExoS possesses FAS (factoractivating exoenzyme S)-dependent ADP-ribosyltransferase activity and iscytotoxic to eukaryotic cells. FAS is a member of the 14-3-3 family ofproteins that regulate the activity of several eukaryotic enzymes. Priorto this invention, no analogues to ExoS have been found in bacteriaother than P. aeruginosa.

SUMMARY OF THE INVENTION

A protein toxin named Aeromonas salmonicida exoenzyme T (AexT), whichbelongs to the family of ADP-ribosylating toxins, was identified andcharacterized in Aeromonas salmonicida, the etiological agent offurunculosis in fish. This discovery has enabled the development ofdiagnostic, preventative, and therapeutic techniques, including thepreparation of traditional or recombinant vaccines based on AexT forinducing immunity against A. salmonicida infections, and including theidentification and characterization by known methods of homologoustoxins and promoters in other Aeromonas species or other bacterialgenera. Also identified and characterized was the Calcium- (or othercation concentration-) dependent promoter of A. salmonicida. This novelpromoter is useful for regulating the expression of proteins inrecombinant expression systems.

In one embodiment, the invention comprises an isolated nucleic acidsegment (SEQ ID NO:1) encoding a 50 kDa exoenzyme of A. salmonicida. Inanother embodiment, the invention comprises a nucleic acid segment thatencodes a protein having the amino acid sequence of SEQ ID NO:2,including variants that retain either biological activity orimmunogenicity or both. Due to the degeneracy of the genetic code andthe possible presence of flanking nucleic acid fragments outside of thecoding region, it will be understood that many different nucleic acidsequences may encode the amino acid sequence of SEQ ID NO:2 andvariants, and that all such sequences would be encompassed within thepresent invention.

In a further embodiment, a method of producing, protecting, capturing,or preserving AexT toxin by growing A. salmonicida on Ca²⁺ or othercations depleted medium is provided. This provides a means of preparinga vaccine that is much more effective than currently available vaccinesagainst A. salmonicida. In another embodiment, the invention relates tothe use of AexT as an immunogen and to the use of AexT in a recombinantor traditional vaccine to reduce the incidence of infection by A.salmonicida.

In another embodiment, the invention comprises an improved bacterinvaccine in which the production of AexT has been induced prior toinactivation of the A. salmonicida cells, or in which A. salmonicida hasbeen manipulated (using recombinant or other means) to constitutivelyexpress AexT prior to inactivation.

In a further embodiment, the invention provides a means of diagnosing A.salmonicida, or other bacteria found to contain AexT homologues, by thedetection of the AexT protein or the homologous proteins. Also, theinvention provides a toxin that in itself may have therapeutic activityin certain unrelated disease states, or for treatment of certainconditions in man or animals.

In a further embodiment, the invention comprises an isolated nucleicacid segment (SEQ ID NO:3) encoding the promoter sequence of the geneencoding the AexT protein. This promoter is regulated by Calcium, andpossibly by other cations as well as other undefined sensory signals,and is useful for regulating the expression of proteins in recombinantexpression systems.

The gene aexT encoding the toxin AexT, was identified by broad rangetoxin gene probes. It was cloned and characterized by sequence analysis.The cloned gene was expressed, and purified AexT was obtained byrecombinant gene technology in E. coli. AexT shows significant sequencesimilarity to the ExoS and ExoT exotoxins of Pseudomonas aeruginosa andto the YopE cytotoxin of Yersiniae sp. Recombinant AexT possessesenzymatic ADP-ribosyltransferase activity. Monospecific polyclonalantibodies directed against purified recombinant AexT cross-react withExoS and ExoT of P. aeruginosa. Secretion of AexT from freshly isolatedstrains of A. salmonicida requires medium depleted of free Ca²⁺ ions (orother cations) or necessitates contact with fish cells, as demonstratedwith cultivated rainbow trout gonad cells. These cells undergosignificant morphological changes upon infection through the toxicactivity of AexT. The dependence on fish cells or on Ca²⁺ (or othercation) restricted conditions for the expression and secretion of theAexT protein toxin by A. salmonicida indicates that regulation ofexpression of the aexT gene and secretion of AexT is coupled to a typeIII secretion system.

The induction of AexT biosynthesis in A. salmonicida is regulatedthrough contact with target cells via a sensory process similar to thatfound in Yersiniae sp. as indicated by the orfX gene flanking the aexTgene. The orfX shows high similarity to the gene for specific Yopchaperon E (sycE) of Yersiniae. SycE serves as a secretion signal and ispart of the type III secretion pathway for secretion of YopE. Culturesof the A. salmonicida type strain ATCC 33658^(T), which were passaged invitro for a large number of generations, and which seem to have lostvirulence, do not produce significant amounts of AexT and do not affectrainbow trout gonad cells morphologically upon infection in spite of thepresence of the aexT gene, in contrast to a fresh field isolate of A.salmonicida. The ADP-ribosylating toxin AexT is a determinativevirulence factor of A. salmonicida and is expected to provide newinsights in basic mechanisms of virulence of this pathogen, andpotentially a protective antigen for vaccination against furunculosis.

SEQUENCE LISTING

The nucleic and amino acid sequences listed in the accompanying sequencelisting are shown using standard letter abbreviations for nucleotidebases, and one letter code for amino acids. Only one strand of eachnucleotide acid sequence is shown, but the complementary strand isunderstood as included by any reference to the displayed strand. Seq ID1 Aeromonas salmonicida gene for aexT complete coding DNA sequence.ATGCAGA TTCAAGCAAA CACCGTCGGC ACACAGGCCG TCGCTCACCA CAGTGATGCCACGACCGGAG TTGGCCGGAT GGGTCAGATG GAGGCGCGTC AGGTCGCCAC CGGACAAGATGCGATCCTGC TGGGCAGTCG CAGCGAACCG CAAAAAGGGC AGGGGCTGCT CTCGCGACTGGGGGCCCAGC TGGCCCGCCC GTTCGTGGCC ATCAAAGAGT GGATCAGCAA CCTGCTGGGGACGGACAAGC GTGCCGCTGC GCCGAAGGCG CAAACCGCCG TTTCCCCCGA GGATCTTCAGCGACTGATGA AGCAGGCTGC ATTTGGTAGC TCGCTGGGTG GCTTCGCCAA GGCGGACGTGTTGAACAACA TCACAGGCGA ACAATTGGGC AAGGATCACG CCAGTCTGGC GACCGGCAATGGCCCCCTGC GCTCTCTCTG CACCGCGTTG CAGGCCGTTG TCATAGGATC TCAGCAACCGCAACTCCGGG AGTTGGCTAC CGGGCTGCTG GCCCGCCCCA TCGCCGGTAT CCCGCTCCAGCAGTGGGGGT CGGTAGGCGG CAAGGTGACC GAGCTGCTCA CCAGCGCCCC CCCCGAACTGTTGAAGGAGG CTATGAGCCA GCTACACACC GCGATGGGTG AAGTTGCCGA CCTGCAGCGCGCTGTAAAGG CAGAAGTTGC TGGCGAACCG GCGCGAAGCG CGACCACAGC GGCCGCTGTGGCGCCGCTCC AAAGCGGTGA GAGCGAAGTT AACGTTGAGC CTGCCGACAA GGCGCTGGCAGAGGGCTTGC AGGAGCAATT CGGCCTGGAG GCCGAGCAAT ATCTGGGTGA ACAGCCCCACGGTACTTACA GCGATGCTGA AGTGATGGCG CTTGGGCTCT ACACCAACGG CGAATACCAGCACCTGAATC GCTCGCTGCG TCAGGAAAAG CAGCTGGATG CAGGGCAAGC GTTGATCGATCAGGGTATGT CCACCGCTTT TGAGAAAAGT ACCCCCACCG AGCAGTTGAT CAAGACCTTCCGCGGTACCC ACGGCGGCGA TGCGTTCAAC GAGGTGGCAG AGGGGCAAGT CGGTCATGATGTCGCTTATC TTTCCACCTC TCGGGATCCC AAGGTGGCAA CCAACTTTGG TGGTTCAGGCTCCATATCCA CGATATTTGG CCGCTCGGGG ATCGATGTCA GCGACATATC CGTTGAAGGTGACGAGCAGG AGATCCTCTA TAACAAAGAG ACTGATATGC GGGTATTGCT CAGTGCCAAAGATGAACGGG GCGTCACCCG GCGGGTACTG GAAGAGGCCT CGCTGGGGGA ACAAAGCGGCCACAGCAAGG GGCTGCTGGA CGGGCTGGAT CTGGCAAGAG GAGCGGGCGG TGCCGACAAGCCGCAAGAGC AAGATATCCG TCTGAAGATG CGCGGGCTCG ATTTGGCGTG A Seq ID 2Aeromonas salmonicida protein sequence for the AexT protein   1MQIQANTVGT QAVAHHSDAT TGVGRMGQME ARQVATGQDA ILLGSRSEPQ  51 KGQGLLSRLGAQLARPFVAI KEWISNLLGT DKRAAAPKAQ TAVSPEDLQR 101 LMKQAAFGSS LGGFAKADVLNNITGEQLGK DHASLATGNG PLRSLCTALQ 151 AVVIGSQQPQ LRELATGLLA RPIAGIPLQQWGSVGGKVTE LLTSAPPELL 201 KEAMSQLHTA MGEVADLQRA VKAEVAGEPA RSATTAAAVAPLQSGESEVN 251 VEPADKALAE GLQEQFGLEA EQYLGEQPHG TYSDAEVMAL GLYTNGEYQH301 LNRSLRQEKQ LDAGQALIDQ GMSTAFEKST PTEQLIKTFR GTHGGDAFNE 351VAEGQVGHDV AYLSTSRDPK VATNFGGSGS ISTIFGRSGI DVSDISVEGD 401 EQEILYNKETDMRVLLSAKD ERGVTRRVLE EASLGEQSGH SKGLLDGLDL 451 ARGAGGADKP QEQDIRLKMRGLDLA* Seq ID 3 Aeromonas salmonicida promoter sequence for the AexTgene TGATG GCTCCAGATT GATGATGGCG CCATTAGAGC AGGTCGCCGC CAGCGGCACTGTTAATGGTG GCTCTCATTT TTTAGCTTTT CGGTCAGCAG GATGGCGCGC CGCGCTCAGTACAAAAATCG CGACCAATCC CGATAGTCCC TGTTGATACC CTCTCCTAGA CTGGCGGCGAAACATCACAA GAAGACAATC ATC Seq ID 4 Aeromonas salmonicida proteinsequence for the OrfX protein   1 MNSLYHAAIH QLFLSLSLPQ PQQEESVTSLQIGELTCHLT EHPADYLLMF  51 TRLEVASGAQ AAAQNLFSQD PCKPVLGFDP DDLTPVLWSRQPLQQLDRAQ 101 IHHQVEQLVS AADELSRW* Seq ID 5 Aeromonas salmonicida genefor orfX complete coding DNA sequence TT ACCACCTGCT TAGCTCGTCAGCGGCAGAGA CCAGTTGCTC CAGCTGGTGA TGGATCTGGG CGCGATCCAG CTGCTGCAACGGCTGGCGAC TCCACAACAC CGGCGTCAGA TCGTCGGGGT CAAAACCCAG AACGGGTTTGCAAGGGTCCT GACTAAACAG GTTTTGCGCG GCGGCCTGGG CGCCGCTAGC TACCTCAAGACGGGTAAACA TCAGCAGATA GTCGGCTGGG TGCTCGGTCA GGTGGCAGGT CAGTTCGCCGATCTGCAGGC TGGTGACGCT TTCCTCCTGC TGCGGCTGAG GAAGCGAGAG GGAGAGAAACAGCTGGTGGA TGGCGGCGTG ATAAAGAGAG TTCAT

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Genetic map of the genes encoding AexT and the ORFX of A.salmonicida in alignment with the corresponding genes of P. aeruginosaand Y. pestis. Maps were constructed from EMBL/GenBank Accession noL27629 for P. aeruginosa ExoS, L46800 for P. aeruginosa ExoT and fromAF053946 for Y. pestis. Due to the high homology found within thevirulence plasmids of Y. pestis, Y. pseudotuberculosis and Y.enterecolitica, AF053946 also represents these Yersiniae sp. Boxes witharrowheads indicate ORFs. Numbers indicate corresponding amino acidpositions. The putative biglutamic acid active site is dashed and thealignment with other ADP-ribosylating toxins is shown at the bottom.Black boxes indicate the transcription activator (ExsA) binding site andblack triangles indicate consensus sequences for the transcriptionpromoter, containing −10 and −35-boxes. Abbreviations used: AS, A.salmonicida; PA, P. aeruginosa; YP, Y. pestis; CP, Clostridiumperfringens; EC, E. coli; VC, V. cholera; AexT, Aeromonas exoenzyme T;ExoS, exoenzyme S; ExoT, exoenzyme T; SycE, specific Yop chaperone E;YopE, Yersinia outer protein E; IOTA, IOTA toxin; LT, heat labile toxin;CT, cholera toxin. Scale on top is given in kb.

FIG. 2. Immunoblot reacted with rabbit serum raised againstnitrilotriacetic acid-purified recombinant AexT-His (Lane 5). Equalamounts of supernatants derived from A. salmonicida strains were mixedwith SDS-loading buffer and loaded onto a 10% SDS-polyacrylamide gel. A.salmonicida strains JF2267 (Lane 1 and 3) and ATCC 33658 (Lane 2 and 4)were grown in standard media (Lane 1 and 2) or in Calcium depleted mediacontaining 10 mM NTA (Lane 3 and 4). The molecular masses of theprestained broad range protein markers (BioLabs, Std.) are indicated inkDa to the left.

FIG. 3. Infection of fish cells with A. salmonicida. RTG-2 cells wereexposed to 100 μl PBS containing no cells (A), A. salmonicida ATCC 33658(B) and A. salmonicida JF2267 (C). Bacteria in panel C seem to beattached to cells and cell debris whereas bacteria in panel B areequally distributed over the whole surface and were observed to befloating in the medium. Pictures were taken after 24 hrs of infectionunder a phase contrast microscope.

DETAILED DESCRIPTION OF THE INVENTION

I. Definitions:

Epitope: An epitope refers to an immunologically active region of animmunogen (most often a protein, but sometimes also a polysaccharide orlipid or other molecule) that binds to specific membrane receptors forantigen on lymphocytes or to secreted antibodies. To generate an immuneresponse to a foreign antigen, lymphocytes and antibodies recognizethese specific regions (epitopes) of the antigen rather than the entiremolecule.

B cell epitope: The region of an immunogen which is recognized by Bcells when it binds to their membrane bound antibody. The B cells whichrecognize that particular region then proliferate and secrete antibodymolecules which are specific for that region of the immunogen. B cellepitopes tend to be highly accessible regions on the exposed surface ofthe immunogen. Stimulation of the immune system by B cell epitopesresults in “humoral” immunity.

T cell epitope: The region (epitope) of an immunogen which is recognizedby a receptor on T cells after being processed and presented on thesurface of an antigen presenting cell (APC) in the context of a majorhistocompatability complex (MHC) class I or II molecule. T cells can besplit into two distinct groups, T helper cells (T_(h)) and T cytotoxiccells (T_(c)). T helper cells recognize epitopes bound to MHC class IImolecules whereas T cytotoxic cells recognize epitopes bound to MHCclass I molecules. T helper cells can be further subdivided into twoclasses, T_(h1) and T_(h2), T_(h2) being responsible for stimulation ofcell-mediated immunity and T_(h2) cells stimulating the humoral arm ofthe immune system. When a given T cell recognizes the epitope-MHCcomplex at the surface of the APC it becomes stimulated andproliferates, leading to the production of a large number of T cellswith receptors specific for the stimulating epitope. Stimulation of theimmune system by T cell epitopes normally results in “cell-mediated”immunity.

Attenuated Bacterial Vaccine: This refers to bacterial strains whichhave lost their pathogenicity while retaining their capacity fortransient growth within an inoculated host. Because of their capacityfor transient growth, such vaccines provide prolonged immune-systemexposure to the individual epitopes on the attenuated organisms,resulting in increased immunogenicity and memory-cell production, whichsometimes eliminates the need for repeated booster injections. Theability of many attenuated vaccines to replicate within host cells makesthem very suitable to induce a cell-mediated immunity. Typically,bacterial strains are made attenuated by introducing multiple definedgene mutations into the chromosome thereby impairing growth in vivo.

Recombinant Vector Vaccine: This refers to the introduction of genes (orpieces of genes) encoding major antigens (or epitopes) from especiallyvirulent pathogens into attenuated viruses or bacteria. The attenuatedorganism serves as a vector, replicating within the host and expressingthe gene product of the pathogen.

Traditional Vaccine: A traditional vaccine is a preparation yieldingprotection from disease based on: an inactivated whole pathogen; anattenuated live pathogen; a closely related organism (live or dead)sharing protective epitopes; a toxin; a chemically modified or heatedtoxin; or a purified or partially purified protein from the pathogen ora closely related organism.

Sequence Identity: Identity between two nucleic acid sequences, or twoamino acid sequences is expressed in terms of the level of identicalresidues shared between the sequences. Sequence identity is typicallyexpressed in terms of percentage identity; the higher the percentage,the more similar the two sequences are.

Sequence Similarity: Similarity between two amino acid sequences isexpressed in terms of the level of sequence conservation, includingshared identical residues and those residues which differ but whichshare a similar size, polarity, charge or hydrophobicity. Sequencesimilarity is typically expressed in terms of percentage similarity, thehigher the percentage, the more similar the two sequences are.

Recombinant: A recombinant nucleic acid is one that has a sequence thatis not normally occurring or has a sequence that is made by anartificial combination of two otherwise separated segments of sequence.This artificial combination is often accomplished by chemical synthesisor, more commonly, by the artificial manipulation of isolated segmentsof nucleic acids, e.g., by genetic engineering techniques.

Oligonucleotide (oligo): A linear polymer sequence of up toapproximately 100 nucleotide bases in length.

Probes and primers: Nucleic acid probes and primers may readily beprepared based on the amino acid and DNA sequence provided by thisinvention. A probe comprises an isolated nucleic acid attached to adetectable label or reporter molecule. Typical labels includeradioactive isotopes, ligands, chemiluminescent agents, and enzymes.Methods for labelling and guidance in the choice of labels appropriatefor various purposes are well known in the art.

Primers are short nucleic acids, preferably DNA oligonucleotides 15nucleotides or more in length. Primers may be annealed to acomplementary target DNA strand, and then extended along the target DNAstrand by a DNA polymerase enzyme. Primer pairs can be used foramplification of a nucleic acid sequence, e.g., by the polymerase chainreaction (PCR) or other nucleic-acid amplification methods known in theart.

Methods for preparing and using probes and primers are well known in theart. PCR primer pairs can be derived from a known sequence, for example,by using computer programs intended for that purpose such as DNAStarLasergene software. One of skill in the art will appreciate that thespecificity of a particular probe or primer increases with its length.Thus, for example, a primer comprising 20 consecutive nucleotides willanneal to a target with a higher specificity than a corresponding primerof only 15 nucleotides. Thus, in order to obtain greater specificity,probes and primers may be selected that comprise 20, 25, 30, 35, 40, 50or more consecutive nucleotides.

Isolated: An “isolated” biological component (such as nucleic acid orprotein or organelle) has been substantially separated or purified awayfrom other biological components in the cell of the organism in whichthe component naturally occurs, i.e., other chromosomal andextra-chromosomal DNA and RNA, proteins and organelles. Nucleic acidsand proteins that have been “isolated” include nucleic acids andproteins purified by standard purification methods. The term alsoembraces nucleic acids and proteins prepared by recombinant expressionin a host cell as well as chemically synthesized nucleic acids. An“isolated” bacterial strain or colony is purified away from othercolonies and yields a pure culture without any contaminants upon platingon selective media.

Vector: A nucleic acid molecule as introduced into a host cell, therebyproducing a transformed host cell. A vector may include nucleic acidsequences that permit it to replicate in a host cell, such as an originof replication. A vector may also include one or more selectable markergenes and other genetic elements known in the art. A“temperature-sensitive” vector is one which replicates normally at a lowgrowth temperature (i.e., 28 C) and will not replicate at a highergrowth temperature (i.e., 42 C) due to mutations at or near the originof replication. An “imperfectly segregating” vector is one which is notstably inherited by new daughter cells at the time of cell division inthe absence of selection pressure due to mutations within the vectorsequence.

Host Cell: Refers to those cells capable of growth in culture andcapable of expressing AexT protein and/or AexT fusion protein. The hostcells of the present invention encompass cells in in vitro culture andinclude prokaryotic and eukaryotic cells, including insect cells. A hostcell strain may be chosen which modulates the expression of the insertedsequences, or modifies and processes the gene product in the specificfashion desired. Expression from certain promoters can be elevated inthe presence of certain inducers (i.e. temperature, small inducermolecules such as Beta-galactosides for controlling expression of T7 orlac promoters or variants thereof). The preferred host cell for thecloning and expression of the AexT protein and AexT fusion protein is aprokaryotic cell. An example of a prokaryotic cell useful for cloningand expression of the AexT protein of the present invention is E. coliBL21 (DE3).

Cell Culture: Refers to the growth of eukaryotic (non-bacterial) cellsin a complex culture medium generally consisting of vitamins, buffers,salts, animal serum, and other nutrients.

Fusion Partner: Any DNA sequence cloned in frame to the 5′ or 3′ end ofan ORF that results in transcription and translation of amino acidsequence added to the N— or C-terminus of the original protein.

Fusion Protein: The term fusion protein used herein refers to thejoining together of at least two proteins, an AexT protein and a secondprotein. In some embodiments of the present invention, the secondprotein may be fused or joined to a third protein. In the presentinvention, examples of second proteins include any polypeptide thatfacilitates the following: expression, secretion, purification,condensation, precipitation, or any property which facilitatesconcentration or purification.

Promoter: A sequence of DNA to which the enzyme RNA polymerase binds(directly or through an intermediary protein, RNA polymerase bindingprotein) before initiation of transcription of the DNA into RNA. Apromoter allows expression of a protein from the DNA coding sequence.

Variant: Any molecule in which the amino acid sequence, glycosylation,phosphorylation, and/or lipidation pattern or any other feature of anaturally occurring molecule which has been modified covalently ornon-covalently and is intended to include mutants. Some of the variantsfalling within this invention possess amino acid substitutions,deletions, and/or insertions provided that the final construct possessesthe desired ability of AexT. Amino acid substitutions in AexT may bemade on a basis of similarity in polarity, charge, solubility,hydrophobicity, hydrophilicity, and/or the amphipathic nature of theresidues involved. Also included within the definition of variant arethose proteins having additional amino acids at one or more of theC-terminal, N-terminal, and within the naturally occurring AexT sequenceas long as the variant protein retains biological activity or thecapability to act as an antigen and hence as a vaccine. Original ResidueConservative Substitutions Ala ser Arg lys Asn gln; his Asp glu Gln asnGlu asp Gly pro His asn; gln Ile leu; val Leu ile; val Lys arg; gln; gluMet leu; ile Phe met; leu; tyr Ser thr Thr ser Trp tyr Tyr trp; phe Valile; leu

Table 1: More substantial changes in functional or other features may beobtained by selecting substitutions that are less conservative thanthose in Table 1, i.e., selecting residues that differ moresignificantly in their effect on maintaining (a) the structure of thepolypeptide backbone in the area of the substitution, for example, as asheet or helical conformation, (b) the charge or hydrophobicity of themolecule at the target site, or (c) the bulk of the side chain. Thesubstitutions which in general are expected to produce the greatestchanges in protein properties will be those in which (a) a hydrophilicresidue, e.g., seryl or threonyl, is substituted for (or by) ahydrophobic residue, e.g., leucyl, isoleucyl, phenylalanyl, valyl oralanyl; (b) a cysteine or proline is substituted for (or by) any otherresidue; (c) a residue having an electropositive side chain, e.g.,lysyl, arginyl, or histidyl, is substituted for (or by) anelectronegative residue, e.g., glutamyl or aspartyl; or (d) a residuehaving a bulky side chain, e.g., phenylalanine, is substituted for (orby) one not having a side chain, e.g., glycine. Variant peptides havingone or more of these more substantial changes may also be employed inthe invention, provided that biological activity or immunogenicity ofAexT is retained.

More extensive amino acid changes may also be engineered into variantAexT peptides. These variant peptides will typically be characterized bypossession of at least 40% sequence identity counted over the fulllength alignment with the amino acid sequence of their respectivenaturally occurring sequences using the alignment programs describedbelow. In addition, these variant peptides will retain either biologicalactivity or immunogenicity or both. Confirmation that an AexT peptidehas biological activity may be achieved using the assay system describedherein. Confirmation that an AexT peptide has immunogenic effect may beachieved by studies showing protection. Following confirmation that theAexT peptide has the desired activity or immunogenic effect, a nucleicacid molecule encoding the AexT peptide may be readily produced usingstandard molecular biology techniques. Where appropriate, the selectionof the open reading frame will take into account codon usage bias of thebacterial, plant or other eukaryotic species in which the AexT peptideis to be expressed.

Inclusion body: Intracellularly confined, insoluble, protein-containingparticles of bacterial cells comprised of either homologous orheterologous proteins. These particles are the reservoirs andconsequence of overproduction of bacterial recombinant proteins.Inclusion bodies can be purified or semi-purified and used directly asprotein antigens or can be solubilized by various procedures and used assoluble protein antigen preparations.

Alignment programs: Methods for aligning sequences for comparisonpurposes are well known in the art. Various programs and alignmentalgorithms are described in Smith and Waterman 1981), Needleman andWunsch (1970), Pearson and Lipman (1988), Higgins and Sharp (1988,1989), Corpet et al. (1988), Huang et al. (1992), Pearson et al. (1994).Altschul et al. (1990) presents a detailed consideration of sequencealignment methods.

The National Centre of Biotechnology Information (NCBI) Basic LocalAlignment Search Tool BLAST; Altschul et al., 1990) is available fromseveral sources, including the NCBI (Bethesda, MD) and on the Internet,for use in connection with the sequence analysis programs BLASTP,BLASTN, BLASTX, TBLASTN, TBLASTX. BLAST can be accessed athttp://www.ncbi.nlm.nih.gov/BLAST/. A description of how to determinesequence identity using this program is available athttp://www.ncbi.nlm.nih.gov/BLAST/blast_help.html.

For comparisons of amino acid sequences of greater than 30 amino acids,the “BLAST 2 sequences” function in the BLAST program is employed usingthe BLASTP program with the default BLOSUM62 matrix set to defaultparameters, (open gap 11, extension gap 1 penalties). When aligningshort peptides (fewer than 30 amino acids), the alignment should beperformed sing the “Blast 2 Sequences” function employing the BLASTPprogram with the PAM30 matrix set to default parameters (open gap 9,extension gap 1 penalties). Proteins having even greater similarity tothe reference sequences will show increasing percentage identities whenassessed by this method, such as at least 45%, at least 50%, at least60%, at least 70%, at least 75%, at least 80%, at least 85%, at least90%, or at least 95% sequence identity.

II. Selection and Creation of Nucleic Acid Sequences Encoding the 50 kDAexT Protein, and use of AexT in Recombinant and Bacterin Vaccines:

a. Growth and Purification of Aeromonas sp. and Plasmid Cloning Vectors.

Aeromonas strains (Table 1) were routinely cultured on blood agar plates(Trypticase soy agar supplemented with 0.1% CaCl₂ and 5% sheep blood) at37° C. except for A. salmonicida which were grown at 20° C. A.salmonicida strain JF2267 was freshly isolated from an arctic char(Salvelinus alpinus) with typical furunculosis symptoms. The strain wasidentified as A. salmonicida by a routine diagnostic agglutination testusing rabbit anti-Aeromonas salmonicida specific antiserum and bysequence analysis of the rrs (16S rRNA) genes (EMBL/GenBank AccessionNo. AF200329) as described by Kuhnert et al.

Escherichia coli strains XL1-blue (recA1 endA1 gyrA96 thi-1 hsdR17supE44 relA1 lac [F′ proAB lac^(q)Z M15 Tn10 (Tet^(r))]^(c)), andBL21(DE3) (F ′dcm ompT hsdS(r_(B)-m_(B)-) gal (DE3)) were used forcloning and expression of cloned genes respectively. PlasmidpBluescriptIISK- (Stratagene, La Jolla, Calif.) was used as cloningvector. Plasmid pETHIS-1 is a T7 promoter based expression vector andallows addition of poly-histidine tails at the N-terminal or at both N—and C-terminal ends of proteins (nucleotide sequence accession no.AF012911). E. coli strains were grown at 37° C. in Luria-Bertani broth(LB) supplemented when necessary with ampicillin (50 μg/ml) forselection and maintenance of recombinant plasmids. For blue-whiteselection with pBluescriptIISK- 125 μM X-Gal(5-bromo-4-chloro-3-indolyl- -D-galactopyranoside) and 1 mM IPTG(isopropyl- -D-thiogalacto-pyranoside) were added.

P. aeruginosa ATCC 27853 was grown for 8 hrs on an LB-plate at 20° C. Inorder to induce ExoS and ExoT secretion cells were then incubated 18 hrsat 20° C. in 20 ml TSB (2.75 g/100 ml Trypticase-Soy broth withoutdextrose, 1% Glycerol, 0.1 M L-Glutamic acid, pH=7.3) supplemented with10 mM NTA (Nitrilo-triacetic acid (Titriplex I) pH 7.3) for chelation ofCa²⁺ ions. Subsequently 5 mM PMSF were added and the culture wascentrifuged for 15 min at 4,000 rpm. This protein fraction was used forfurther analyses on Western blots and for activity assays.

A. salmonicida ATCC 33658¹ was cultured at 20° C. in various mediaconsisting of TSB supplemented with either 10 mM CaCl₂, or 0.01 to 1 mMEDTA, or 1 mM EDTA +1 mM PMSF, or 10 mM NTA, or 100 mM NTA, or 0.1 mMFeIII-Cl₃, or 0.1 mM FeII-Cl₂, or 0.1 mM EDDA (Ethylendiaminedi(o-hydroxy-phinyl-acetic-acid)), or 20 mM Na-oxalate. A temperatureshift experiment was performed by cultivating A. salmonicida ATCC33658^(T) in TSB at 20° C. to an OD₆₀₀ of 0.1 followed by furtherincubation over night at 32° C. Additionally various other A.salmonicida strains were cultured using the same conditions as describedfor P. aeruginosa. Culture supernatants and total cell protein extractswere analysed on western blots.

b. PCR, Cloning and Preparation of Gene Probes for ADP-RibosylatingToxins.

PCR were carried out with a DNA thermal cycler (GeneAmp 9600; PEBiosystems, Norwalk, Conn.) in 50 μl reaction mixes containing 10 mMTris-HCl, pH 8.3, 1.5 mM MgCl₂, 50 mM KCl, 0.25 μM forward and reverseprimers, 0.5 units Taq polymerase, and 5 ng template DNA. The DNAs wereamplified for 35 cycles with 30 s denaturation at 94° C., 30 s annealingat corresponding temperatures (Table 2), and 1 min extension at 72° C.For fragments above 1 kb, the extension time was extended by 1 min perkb. When DNA fragments were produced by PCR for subsequent cloning andexpression, the expand long template PCR kit (Roche MolecularBiochemicals, Rotkreuz, Switzerland) containing polymerase withproof-reading capacity was used instead of Taq polymerase. In addition,an extension step of 7 min at 72° C. was added at the end of the lastcycle in order to ensure full length synthesis of the differentfragments. For the production of DIG-labelled probes PCR mixtures weresupplemented with 40 μM digoxigenin-11-dUTP (Roche MolecularBiochemicals).

A DNA fragment (called REXOS) corresponding to the catalytic portion ofthe P. aeruginosa exoS gene was amplified with the primer-pair REXOS-Land REXOS-R (Table 2) both containing EcoRI restriction site linkers.When genomic DNA of P. aeruginosa ATCC 27853 was used as template forPCR, 10% dimethyl sulfoxide were added. PCR-fragments were purified withthe QIAquick PCR purification kit (QIAGEN, Basel, Switzerland). PlasmidpBluescriptIISK- and purified PCR fragments were digested with EcoRI andligated for 2 hrs at room temperature before transformation of E. coliK-12 strain XL1-blue. After blue-white screening a white colony wasselected and the prepared plasmid was sequenced in order to excludecloning artefacts.

To get pure, plasmid-contaminant free probes, the cloned exoS derivedfragment (REXOS) was excised with EcoRI, purified twice over agarosegels with the Jet-Sorb kit (Genomed GmbH, Bad Oeynhausen, Germany), andthen used as template for PCR with the primers REXOS-L and REXOS-R(Table 2) for production of the DIG-labelled probe REXOS.

A DNA fragment (called RASEXOS) corresponding to the putative catalyticportion of A. salmonicida aexT gene was amplified with the primer-pairRASEXOS-L and RASEXOS-R (Table 2) and DIG-labelled. Genomic DNA derivedfrom A. salmonicida ATCC 33658^(T) (type strain) served as template.

All cloning procedures were essentially performed using standardprotocols. DNA was extracted by the method of Pitcher et al. andmanipulated using conventional methods. The CaCl₂ method was used forpreparation of competent cells. Sequencing reactions were performed witha Taq Dye Deoxy Terminator cycle sequencing kit (PE Biosystems) andreaction products were analysed on an ABI Prism 310 genetic analyser (PEBiosystems).

Amplification of a DNA fragment containing the intergenic orfx-aexTregion with the putative promoter region of aexT was performed by PCRusing the primer pair BASEXOS693 and BASEXOS-250 (Table 2), genomic DNAof either the A. salmonicida ATCC 22658^(T) or JF2267 strain as templateand the Pwo DNA Polymerase (Roche Molecular Biochemicals). Theintergenic region of the two genes orfx and aexT was subsequentlysequenced with primer BASEXOS-250.

c. Construction of A. salmonicida Lambda Phage—Gen Library.

Genomic DNA (0.1 μg) from A. salmonicida ATCC ₃₃₆₅₈ ^(T) was digestedpartially with the restriction enzyme Sau3a to get fragments in the 3 to4 kb range which were ligated to ZapExpress digested with BamHI(Pharmacia LKB, Biotechnology AB, Uppsala, Sweden) and packed intoLambda. A fresh culture of 200 μl of E. coli XL1-blue MRF′ (PharmaciaLKB) was resuspended in 10 mM MgSO₄ at an OD₆₀₀ of 1.0 and infected withthe packed phages during 20 min at 37° C. Five ml Top Agarose (LB-brothsupplemented with 0.7% Agarose) supplemented with IPTG and X-Gal wereadded, gently mixed and immediately poured onto an LB-Agar plate. Plateswere incubated over night at 37° C. and plaques were lifted on nylonfilters and screened using DIG-labelled probes. Positive plaques werecut out and stored over night at 4° C. in 0.5 ml SM-buffer (100 mM NaCl,8 mM MgSO₄, 50 mM Tris pH 7.5 and 0.01% gelatine) containing 20 μlchloroform. The in vivo excision of plasmids from selected phagemidplagues was done according the instructions of the ZAP express kit(Pharmacia LKB). Colonies with plasmids containing cloned fragments wereisolated and mini-preps (Quiagen) were performed for plasmidpurification.

d. Sequence Data Analyses.

Sequence alignment and editing were done with the software Sequencher(Gene Codes Corporation, Ann Arbor, Mich.). Sequence comparisons weredone as described by Altschul et al. and sequences were aligned with theWiscosin Package (Genetics Computer Group, Inc. [GCG], Madison, Wis.).The theoretical isoelectric pH (pI) and molecular masses of protein werecalculated with the GCG software.

e. Southern Blot Analyses.

Southern blotting was done by alkaline transfer onto positively chargednylon membranes (Roche Molecular Biochemicals) with an LKB 2016 VacuGenevacuum blotting pump (Pharmacia LKB). Gels were depurinated for 10 minin 0.25 M HCl, and subsequent transfer was performed with 0.4 M NaOH.After blotting, filters were baked for 30 min at 80° C. under vacuum.After at least 1 h of prehybridisation, hybridisation was carried out in5×SSC (1×SSC is 0.15 M NaCl plus 0.015 M sodium citrate)-1% blockingreagent (Roche Molecular Biochemicals)-0.1% N-lauroylsarcosine sodiumsalt-0.02% sodium dodecyl sulphate (SDS) at 68° C. over night, usingDIG-labelled DNA fragments as probe. Filters were washed undernonstringent conditions twice for 5 min each with 50 ml of 2×SSC-0.1%SDS per 100 cm² at room temperature (20° C.), followed bymedium-stringency washing twice for 15 min each with 50 ml of0.2×SSC-0.1% SDS per 100 cm² at 20° C. The filters were then processedwith phosphatase-labelled anti-DIG antibody according to the producer'sprotocol. Signals were produced with chemiluminescent substrate(CDP-Star; Roche Molecular Biochemicals) or with the chromogenesubstrate NBT-BCIP. Luminescence was detected on X-ray films.

f. Expression of His-Tailed Fusion Proteins.

E. coli BL21 (DE3) cells harboring recombinant pETHIS-1 plasmids withcloned genes were inoculated in 50 ml of LB-ampicillin at 37° C. to anOD₆₀₀ of 0.3 and induced by addition of 0.2 mM IPTG (final conc.) andsubsequent growth for 3 hrs. The cells were sedimented by centrifugationat 4,000 rpm, resuspended in 5 ml of buffer pH 7.9 containing 10 mMTris-HCl, 1 M Urea, 250 mM NaCl, 2.5 mM Imidazole, 3 M Guanidium HCl,0.2 mM PMSF and sonicated with a microtip for 20 min at 50% and 1-sinterval in a Branson Sonifier 250 (Branson Ulatrasonics, Danbury,Conn.). This sonicated fraction was directly loaded onto a prewashed1.25-ml-bed-volume Ni-NTA column (Quiagen) and washed once more with 5ml binding buffer (2 M Urea, 20 mM Tris, 500 mM NaCl, 5 mM Imidazole, 60mM Guanidium HCl pH 7.9).

Elution of the poly-histidine tailed proteins was performed with a 40-mlbinding buffer-to-elution buffer (2 M Urea, 20 mM Tris, 500 mM NaCl, 500mM Imidazole, 60 mM Guanidium HCl, pH 7.9) gradient with a flow rate of0.25 ml/min and collection of fractions of 1 ml with a HiLoad system(Pharmacia LKB). The fractions were analysed on SDS-10% acrylamide gels.Those containing the purified fusion protein were pooled and dialysedover night against 5 liters of 0.85% NaCl.

g. Immunisation of Rabbits with Purified Proteins.

Purified and dialysed recombinant protein solution (100 μg/ml) was mixed1:1 with complete Freund's adjuvant (Difco Laboratories, Detroit Mich.)and 2 ml of the emulsion were then injected subcutaneously to a rabbit.The rabbit was booster immunised with the same amount of proteinemulsified with Freund's incomplete adjuvant 21 days later. On day 45after the first immunisation, the rabbit was bled, and blood serum wasprepared and stored at −20° C.

h. Infection of Fish Cell Cultures with A. salmonicida.

Rainbow trout (Oncorhynchus mykiss) gonad cells (RTG-2, ATCC CCL-55)were grown in 75 cm² tissue culture flasks (Techno plastic products AG,Trasadingen, Switzerland) at 22° C. in minimum essential medium(GibcoBRL Life Technologies, Basel, Switzerland) supplemented with 2 mML-glutamine (GibcoBRL), 1× non essential amino acids (GibcoBRL), 3 g/lsodium bicarbonate and 10% foetal bovine serum. Three days beforeinfection the cells were trypsinised and 4 million cells were seededinto a 25 cm² tissue culture flask. Monolayered RTG-2 cells wereinfected at a multiplicity of infection of 10:1 (bacteria:fish cells)with cells of A. salmonicida cultures resuspended in phosphate bufferedsaline (PBS) pH 7.4. As control 100 μl of pure PBS pH 7.4 were added.After 24 hrs of infection at 15° C. the fish cells were photographedunder a green filtered phase contrast microscope (Axiovert 100, Zeiss,Jena, Germany). Detachment of the cells from the flask was obtained byshaking them by hand. The suspended cells were centrifuged for 5 min at4,000 rpm. Lysis of the fish cells was performed in 100 μl distilledwater with two subsequent freeze thawing steps and verified bymicroscopy. The lysed fish cells were used for further analyses onWestern blots and for activity assays.

i. SDS-PAGE and Immunoblot Analyses.

Proteins were separated by SDS-10% polyacrylamide gel electrophoresis(SDS-PAGE) as described by Laemmli and transferred to a nitrocellulosemembrane (Bio-Rad laboratories, Hercules, Calif.). For immunoblotting,Western blots were blocked with 1% milk buffer for 30 min and thenincubated with the rabbit antiserum (1:1500) or with sera (1:100)derived from diseased fish in milk buffer overnight at 4° C. After athorough wash with water, the appropriate phosphatase-labelled conjugate(Goat anti-Rabbit IgG (H+L) [cat. no. 075-1506] or Goat anti-TroutImmunoglobulin [cat. no. 05-29-05], Kirkegaard & Perry, Gaithersburg,Md.) diluted 1:2000 or 1:500 respectively in milk buffer was added, andthe reaction was visualised 90 min later by incubation with BCIP-NBT asthe substrate.

j. ADP-Ribosyltransferase Assays.

ADP-ribosyltransferase assays contained 100 μM ¹⁴C-NAD (specificactivity: 6 Ci/mol) and 0.2 M NaAc pH 6 in a total of 20 μl. Either 0.1mM soy bean trypsin inhibitor (SBTI, Roche Molecular Biochemicals) and50 μg ml⁻¹ wheat germ extract (Promega Corporation, Madison, Wis.) assource of FAS or 4 μl (approximately 200,000 cells) of non infected RTG2fish cells were added as substrate. The reaction was started by adding 4μl aliquots of supernatants of either P. aeruginosa ATCC 27853, A.salmonicida ATCC 33658 or A. salmonicida JF2267. An aliquot of puregrowth medium was used for background determination. The reaction wasperformed at 20° C. for 1 hr and stopped by addition of 500 μl 10%trichloro acetic acid (TCA). The mixtures were blotted onto filters (GS0.22 μm, Millipore, Bedford, Mass.) using a vacuum pump and washed 5times with 0.75 ml 10% TCA. The filters were air dried and scintillationliquid (Emulsifier scintillator plus, Packard instrument company,Meriden, Conn.) was added. Scintillation was detected as counts perminute (CPM) on a liquid scintillation counter (Wallac 1410, Pharmacia,Dübendorf, Switzerland). Experiments were performed in triplicate andscintillation was counted three times per experiment. Background countswere subtracted and results with their standard deviations are given inCPM (Table 3). Due to high background ADP-ribosyltransferase activity ofthe fish cells the activity of AexT in infected fish cells could not bemeasured.

EXAMPLES

The following examples are included to demonstrate preferred embodimentsof the invention, and it will be appreciated by those skilled in theart, in light of this disclosure, that many changes can be made in thespecific embodiments disclosed without departing from the scope of theinvention.

1. Cloning and sequence analyses of aexT and its promoter. Analyses ofdifferent Aeromonas sp. with broad-range ADP-ribosylating toxin probesrevealed a signal for a potential ADP-ribosyltransferase gene for A.salmonicida with probe REXOS which is derived from the catalytic domainof ExoS. This probe was used to screen a Lambda Phage gene library of A.salmonicida ATCC 33658^(T). Three positive overlapping clones were foundand joined together to a continuous DNA fragment of 2260 bp in length.The derived DNA sequence of this fragment revealed a complete ORF of1428 bp showing high similarity with ExoT of P. aeruginosa. In analogyto ExoT, it was called Aeromonas exoenzyme T (AexT) and itscorresponding gene aexT. The cloned fragment contains a further openreading frame, named ORFX which shows similarity to the sycE gene ofYersinia sp. and to ORF1 which precedes exoS of P. aeruginosa (FIG. 1).ORFX is preceded by a RBS and followed by a putative rho-independenttranscription termination site. The sequenced DNA fragment encoding AexTand ORFX showed a high G+C content of 60%, which is above the averageG+C content of A. salmonicida of 55%. The ORF representing aexT containsan ATG initiation codon and TGA stop codon. The 87 bp preceding the ATGshow 71% identical nucleotide positions to the sequence preceding exoSand exoT in P. aeruginosa. The putative ribosomal binding site (RBS),AGAAG, is positioned 10 bp upstream of the ATG. The putative promotersequences −10 box (TAGACT) and the canonical −35 box (CCGATA) of aexTare located at the same positions as those for exoS and exoT. Upstreamof the promoter −10 and −35 box sequences there is a consensus bindingsite (TACAAAAA) similar to the one found upstream of exoS and exoT whichis known in P. aeruginosa to be bound by the transcriptional regulatorExsA. An inverted repeat (AACGGACACCCtcGGGTGTCCGTT) is located 25 bpdownstream of the stop codon of the aexT gene. It has the same stemsequence (CGGACAC) as inverted repeats of the putative transcriptiontermination sites of exoS and exoT.

2. Structural analyses of the AexT. The amino acid sequence for AexT wasdeduced from the nucleotide sequences using the universal genetic code.AexT has a calculated pI of 5.13 and a molecular mass of 50.1 kDa. Blastsearches revealed similarity of AexT with ExoT and ExoS over the wholelength. In addition similarity with the YopE cytotoxin of Yersiniapseudotuberculosis (EMBL/GenBank Accession No. P08008), Y. pestis (Acc.No. P31493) and Y. enterocolitica (Acc. No. M34280) was found within theN-terminal 210 amino acids of AexT (FIG. 1). Gap comparisons with theamino acid sequence of AexT with ExoT and ExoS revealed AexT to beidentical in 62.8% with ExoT (57.9% with ExoS) and similar in 67.5% withExoT (62.8% with ExoS) of the positions (FIG. 1) with a gap of 25 aa inlength, which separates the N-terminal from the C-terminal domain. Gapcomparisons of ExoT with ExoS showed them to be identical in 75.1% andsimilar in 77.7% of the positions. The N-terminal domain of AexTrevealed 33.5% identical and 37.4% and similar amino acid positionscompared to the cytotoxin YopE of Y. pseudotuberculosis and 26.8%identical and 32.8% similar amino acids to YopE of Y. pestis (FIG. 1).The biglutamic acid active site (GDEQEILYNK) found for variousADP-ribosylating toxins is also conserved within the C-terminal domainof AexT (FIG. 1).

3. Specificity of aexT genes for A. salmonicida. Southern blot analysesof genomic DNA of various Aeromonas sp. (Table 1) with DIG labelledprobe for aexT (RASEXOS) revealed a single copy of aexT with a sizeestimated to be approximately 3 kb for all A. salmonicida strains tested(Table 1). None of the other analyzed Aeromonas strains showedhybridisation signals with the aexT probe under these hybridisationconditions, but this does not rule out the possibility that otherstrains of Aeromonas or other bacterial genera may possess homologues ofaexT.

4. Production and characterisation of recombinant AexT. In order tocharacterize biochemically the AexT protein and to produce polyclonal,monospecific antibodies directed against AexT, we have expressedpoly-histidine tailed AexT, named AexT-His, in recombinant E. coli K-12strains. The entire coding part inclusive the stop codon of the aexTgene was amplified by PCR using primers BASEXOSH8L and BASEXOSH8R andgenomic DNA of A. salmonicida as template. The purified PCR product wasdigested with restriction enzymes EcoRI and SpeI and cloned into EcoRIand SpeI digested vector pETHIS-1 to obtain plasmid pJFFASAexT-His,encoding N-terminally poly-histidine tailed AexT (AexT-His) under thecontrol of the T7 promoter. For the expression of the aexT-His gene,plasmid pJFFASAexT-His was transformed into E. coli strain BL21(DE 3) asdescribed in Materials and Methods. Biochemical analysis of purified andrenatured recombinant AexT-His revealed that it possessedADP-ribosylating activity (Table 3). Monospecific polyclonal antibodiesagainst AexT were obtained by immunisation of a rabbit with purifiedAexT-His protein as described for other poly-histidine tailed proteins.Anti-AexT antibodies reacted on immunoblots with purified AexT-His. Italso cross reacted with the 49 kDa and 53 kDa proteins in supernatantfrom a culture of P. aeruginosa ATCC 27853, representing the ExoS andExoT protein toxins as expected from sequence similarities with AexT.

5. Expression of AexT in A. salmonicida. The expression of AexT by A.salmonicida type strain ATCC 33658^(T), which seems to have lost itspathogenicity, and of A. salmonicida field strain JF2267, which wasfreshly isolated from a diseased arctic char and which still possessesits virulence, was assessed by immunoblots with anti AexT-Hisantibodies. Neither supernatants nor the cell pellets of the type strainATCC 33658^(T) grown under various conditions showed any specificreactions on immunoblots with monospecific, polyclonal anti-AexTantibodies. In contrast supernatants and cell pellets of A. salmonicidastrain JF2267 grown in TSB supplemented with 10 mM NTA reacted onimmunoblots with anti-AexT-His antibodies (FIG. 2). When NTA was omittedin the growth media, AexT protein in strain A. salmonicida JF2267 onlywas found in the cell pellet but not in supernatants. When A.salmonicida strain JF2267 was analysed during infection of RTG-2 fishcells, a specific reaction on immunoblots using anti-AexT-His antibodieswith a 56 kDa protein corresponding to AexT was found. This indicatesthat strain JF2267 required contact with fish cells or depletion of Ca²⁺ions (or other cations) to induce the production or protection of AexT.However, no AexT could be detected for A. salmonicida type strain ATCC33658 under the same conditions (FIG. 2). ADP-ribosyltransferaseactivity was determined in culture supernatants of A. salmonicidastrains and as control of P. aeruginosa, grown under Ca²⁺ depletedconditions. A. salmonicida field strain JF2267 showedADP-ribosyltransferase values slightly above background and no activitycould be measured in A. salmonicida ATCC 33658^(T), while P. aeruginosashowed high activity (Table 3). ADP-ribosyltransferase could not bedetermined in infected fish cells due to high background activity.

Infection of RTG-2 cells with freshly cultured A. salmonicida strainJF2267 caused a toxic effect showing characteristic cell rounding,detachment and lysis of cells within 24 hours (FIG. 3C) whereas thecells infected with A. salmonicida type strain ATCC 33658^(T) (FIG. 3B)or the control cells incubated with pure PBS (FIG. 3A) showed nomorphological changes at all. Infection of fish cell culture with A.salmonicida JF2267 also induced the production of the AexT protein whichreacted with anti-AexT-His antiserum. Similar morphological changes havebeen reported for cells infected with ExoS producing P. aeruginosa. Thesera raised against AexT-His also showed cross-reactivity with ExoS andExoT produced by P. aeruginosa. Similar cross-reactivity was found foranti-exoenzyme S IgG which reacted with both ExoS and ExoT. The factthat AexT is produced specifically in contact with fish cells (FIG. 3C)or in Ca²⁺ depleted medium (FIG. 2), which is believed to act aspseudo-trigger to induce aexT, suggests the protein to be producedspecifically during infection and hence to play an important role inpathogenicity. Interestingly, A. salmonicida type strain ATCC 33658^(T)does not affect the morphology of RTG-2 cells. It seems to have lost theability of producing cell contact induced AexT probably due to repeatedpassages on growth medium.

In order to determine whether the significant differences in AexTproduction and toxic effect between A. salmonicida isolate JF2267 andtype strain ATCC 33658 could be due to mutations within the putativepromoter regions of their respective aexT genes, the intergenic regionsbetween orfX and aexT were sequenced and found to be identical. Thus,the alteration responsible for the loss of AexT production in the typestrain seems to reside outside the aexT operon. Nevertheless, both A.salmonicida strains ATCC 33658^(T) and JF2267 have the same haemolyticactivity as estimated on blood agar plates implying that the toxiceffect for RTG-2 cells is not due to the A. salmonicida haemolysins butrather to production of AexT in strain JF2267. The loss of expression ofaexT as observed in A. salmonicida type strain ATCC 33658 is a frequentevent in this species, and may explain the currently observed variationsin virulence and also differences in efficacy of protection of wholecell antigen vaccines.

6. Recombinant AexT Vaccine Trial (See Appendix A)

7. Testing of different A. salmonicida bacterin vaccines in Altanticsalmon (Salmo salar) (See Appendix B)

The current data indicate that AexT of A. salmonicida is a determinativevirulence factor of this pathogen. While particular elements,embodiments and applications of the present invention have been shownand described, it will be understood, of course, that the invention isnot limited thereto, since modifications may be made by those skilled inthe applicable technologies, particularly in light of the foregoingdescription. The appended claims include within their ambit suchmodifications and variants of the exemplary embodiments of the inventiondescribed herein as would be apparent to those skilled in the applicabletechnologies. TABLE 1 Aeromonas strains used aexT positive_(b)/ SpeciesStrain_(a) strains tested A. salmonicida ATCC 33658_(T) 1/1 A.salmonicida JF2267_(c) 1/1 A. salmonicida field isolates 10/10 A.bestiarum CDC 9533-76 0/1 A. bestiarum field isolates 0/2 A. caviae ATCC15468 0/1 A. caviae field isolates 0/3 A. encheleia DSM 11577 0/1 A.eucrenophila NCMB 74 0/1 A. eucrenophila field isolate 0/1 A. hydrophilaATCC 7966 0/1 A. hydrophila field isolates  0/15 A. jandaei ATCC 495680/1 A. media ATCC 33907 0/1 A. schubertii ATCC 43700 0/1 A. schubertiifield isolate 0/1 A. sobria CIP 7433 0/1 A. trota ATCC 49657 0/1 A.trota field isolate 0/1 A. veronii ATCC 35624 0/1 A. veronii fieldisolates 0/4_(a)ATCC, American Type Culture Collection, Rockville, Md.; J F, JoachimFrey, University of Berne, Switzerland; CDC, Center for Disease Control,Atlanta, Georgia; DSM, Deutsche Sammlung von Mikroorganismen undZellkulturen GmbH, Braunschweig, Germany; NCMB, National Collection ofMarine Bacteria, Aberdeen, Scotland; CIP, Collection of the InstitutPasteur, Paris, France._(b)Determined by Southern blotting using DIG labelled RASEXOS as probe._(c)JF2267 was isolated freshly from an arctic char with typicalsymptoms of furunculosis. Identification was done phenotypically and by16s rDNA gene sequencing.

TABLE 2 Oligonucleotide primers Annealing temp Name Sequence^(a) 5′ to3′ Position ° C. EXOS-L cgcgaattcACTGGCTGGGCAAACTG 1128-1144_(b) 52EXOS-R cgcgaattCCCGCTGACATCGATTC 2034-2019_(b) 52 RASEXOS-LGGCGGTTGGGCTCTACAC 1537-1554_(c) 60 RASEXOS-R GAGCCCGCGCATCTTCAG2089-2072_(c) 60 BASEXOSH8L cgcgaattCGGCGAAACATCACAAGA  645-662_(c) 59BASEXOSH8R ggactagTCCCGCCAGCATAAAAAAC 2165-2147_(c) 59 BASEXOS693AGGCTCAACGTTAACTTCGC 1432-1413 58 BASEXOS-250 AGAGGGAGAGAAACAGCTGG 427-446 58^(a)Lowercase letters indicate nucleotides added to create restrictionenzyme recognition sites (underlined) for cloning._(b)Based on nucleotide sequence L27629 of P. aeruginosa_(c)Based on nucleotide sequence of A. salmonicida

TABLE 3 Determination of ADP-ribosyltransferase activity cpm^(A)) std.dev.^(C)) A. salmonicida ATCC 33658^(T), culture supernatant 0 ±10JF2267, culture supernatant 5 ±12 AexT-His 123  ±11 P. aeruginosa ATCC27853 culture supernatant (ExoS + ExoT) 4286^(B))   ±125^(A))Mean values corrected for background. Experiments were performed intriple and scintillation was measured three times per experiment.^(B))The proportion of ADP-ribosyltransferase activity of ExoT isestimated to be approximately 0.2% corresponding to 8 cpm under theseconditions.^(C))Standard deviation in cpm

APPENDIX A

Recombinant AexT Vaccine Trial

Materials:

Vaccine Formulations:

-   -   1. The AexT vaccine was formulated using recombinant,        Histidine-tagged AexT resuspended in 10 mM phosphate buffer, pH        7.0, to 200 μg/mL. Four parts of this protein solution were        mixed with one part oil adjuvant for a final AexT concentration        of 150 μg/mL. The dose for testing was 0.1 mL, or 15 μg/fish.    -   2. The commercial comparator vaccine was serial 4-13 of the        vaccine MultiVacc4 (Bayotek International Ltd.)    -   3. The placebo (control) vaccine consisted of phosphate buffered        saline(PBS) (10 mM phosphate, 150 mM NaCl, pH 7.2).    -   4. All vaccines were maintained at 4° C. until use.        Methods        Trial Design:

Fish (rainbow trout Oncorhynchus mykiss) that have been determined to bepathogen free and are at least 15 g in size are held for at leastone-week pre vaccination for acclimation purposes. During theacclimation period the fish are offered 1% body weight in salmonid fishfood every day, however they are denied food 24 hours pre andpost-vaccination.

At least 50 fish are vaccinated 0.1 mL of AexT vaccine viaintra-peritoneal (IP) injection, or 0.2 mL of the commercial vaccineMultiVacc4. At the same time a group of at least 50 fish from the samestock are mock vaccinated with 0.1 mL of PBS. Vaccinated fish are thenheld for a period of at least 350-degree days to allow specific immuneresponse generation in an acclimation tank with a continuous flow ofwater at a temperature of 12-13° C. The fish are offered 1% body weightIn salmonid fish food daily until 24 hours pre-challenge andpost-challenge.

After at least 350-degree days post vaccination 50 fish per group werechallenged by IP injection with a pre-determined concentration ofvirulent Aeromonas salmonicida. The dosage depends on the source of thefish and the water temperature (this is determined empiricallyimmediately prior to challenge of test fish). The identical procedure isperformed with the placebo vaccinated control fish. The fish areobserved daily for mortality for 21 days post challenge and the cause ofmortality assessed and examined to ensure that mortality Is attributedto the challenge organism. After 24 hours post-challenge the fish areagain offered 1% body weight In salmonid fish feed daily. Tanks aremaintained with a continuous flow of water at a temperature of 12-13° C.For a challenge series to be considered satisfactory; all challengegroups must meet the following criteria:

-   -   1. At least 70% of the non-immunized controls must die within 21        days of challenge.    -   2. A relative percent survival (RPS) of no less than 25% must be        achieved for the challenge disease before a vaccine is        considered even partially efficacious for this disease.        RPS=[1−(% mortality vaccinates/% mortality controls)]×100    -   Specificity of immunity was confirmed by challenge with Vibrio        anguillarum

Developed from: The Rules Governing Medicinal Products in the EuropeanUnion, Volume VII, Guidelines for the testing of veterinary medicinalproducts. 1994. Specific Requirements for the Production and Control ofLive and Inactivated Vaccines Intended for Fish. Section 3.2. Potency.

Results Group % Mortality RPS PBS 82 — AexT 37 55 MultiVacc4 30 63

-   -   1. There was a strong challenge with 82% control mortalities.

Vibrio anguillarum immersion challenge shows that the AexT protectsspecifically against A. salmonicida (93% mortality In AexT vaccinatescompared to 15% for commercial vibrio vaccine vaccinates). Challengedsurvivors of the A sal challenge (and salines) with Vibrio anguillarumtype 1. The challenge organism used was Vibrio anguillarum serotype 01at an O.D. of 0.5 (˜8.0×10E8 CFU/mL). This indicates that the immuneresponse is specific.

APPENDIX B

Testing of Different Aeromonas salmonicida Bacterin Vaccines in AtlanticSalmon (Salmo salar)

Purpose: Determination of the efficacy of Aeromonas salmonicidabacterins produced by different methods in Atlantic salmon (Salmosalar), and a correlation between protection and AexT production.

Materials:

Methods:

A. salmonicida vaccine preparations:

-   -   1. Bacterin Preparations: A standard A. salmonicida vaccine        master working seed from Microtek International (1998) Ltd.,        MSW26, was used for all vaccine preparations. The starter        culture for each fermentation was derived from 25 mL of Tryptic        Soy Broth inoculated with a single 1 mL frozen (−80° C.) aliquot        of MSW26 followed by incubation with shaking (18° C. at 100 rpm)        for 36 hours. This primary starter culture was used to Inoculate        a 10 L fermenter        -   a. Bacterin 1: Bacterin 1 was prepared by fermented culture            incubated at 20° C. ±2° C., with sufficient agitation and            aeration to maintain the dissolved oxygen (DO₂) at above            approximately 25%. The pH is maintained between 6.5 and 7.5            (pH controlled by the addition of a NH₄OH solution and            aqueous KOH). The media is Tryptic soy broth with glucose at            1%. During fermentation a concentrated sterile solution of            glucose is fed into the fermenter. Glucose is fed to            maintain a glucose concentration of between 1.0 g/l to 10.0            g/l. Concentrated, sterile solutions of TSB without glucose            are also fed at 1-2% of the culture volume into the            fermenter. The TSB without glucose is fed at OD_(650 nm) of            between 2 and 3, again at OD_(650 nm) of between 4 and 5.            The fermenter culture is fed periodically with a            concentrated sterile solution of glucose to maintain a            glucose concentration of between 1.0 g/l to 10.0 g/l until            the OD_(650 nm) reaches approximately 8.0. The culture is            fed with a concentrated solution of TSB without glucose to            an OD_(650 nm) of approximately 10-12. The pH is maintained            between 6.5 and 7.5 (pH controlled by the automatic addition            of KOH and H₂SO₄). The A. salmonicida culture is inactivated            by the addition of 7.0 mL/l±0.7 mL/l formalin. Aeration to            the fermenter is stopped. The inactivated culture is            agitated in the fermenter for a period of 1 hour at 20°            C.±2° C. The inactivated bacterial culture will then be            pumped or gravity fed directly from the fermenter into            holding vessels and stirred for a further 24 hour            inactivation step. The inactivated bacterial culture was            then held at 4° C. for further processing and formulation.        -   b. Bacterin 2 and 3: Bacterins 2 and 3 were prepared as            duplicates by fermented culture incubated at 20° C.±2° C.,            with sufficient agitation and aeration to maintain the            dissolved oxygen (DO₂) at approximately 15-25%. The pH is            maintained between 6.9 and 7.1 (pH controlled by the            addition of a NH₄OH solution and H2SO4). The media is            Tryptone (1.5%), Yeast extract (0.5%), Glycerol (1%), NaCl            (0.5%), Glutamate (100 mM), and Citrate (20 mM). During            fermentation a concentrated sterile solution of Tryptone            (15%), Yeast extract (5%), NaCl (0.5%), Glycerol (10%),            Glutamate (100 mM), and Citrate (20 mM) is fed into the            fermenter. The fermenter culture is fed continuously with            this concentrated sterile solution to maintain a stable pO₂.            The resulting A. salmonicida culture is inactivated by the            addition of 7.0 mL/l±0.7 mL/l formalin. Aeration to the            fermenter Is stopped. The inactivated culture is agitated in            the fermenter for a period of 1 hour at 20° C.±2° C. The            Inactivated bacterial culture will then be pumped or gravity            fed directly from the fermenter into holding vessels and            stirred for a further 24 hour Inactivation step. The            inactivated bacterial culture was then held at 4° C. for            further processing and formulation.    -   2. Vaccine Formulations: Vaccines were formulated using washed        cells (performed by centrifugation at 1500×g) resuspended in 10        mM phosphate buffer, pH 7.0, to 10 O.D.₆₅₀. Each test vaccine        based on the bacterins 1 through 3 were formulated by mixing 1        part adjuvant with 5 parts washed bacterin cells and 4 parts 10        mM phosphate pH 7. All vaccines were maintained at 4° C. until        use.    -   3. Western immunoblotting: Prior to electrophoresis bacterin        samples containing equivalent cellular mass were solubilized by        mixing with equal amounts of sample loading buffer followed by        boiling for 5 minutes. The prepared samples were then separated        on a 12% polyacrylamide gel by the discontinuous gel method of        Laemmli (1970). Two Identical gels were prepared. One gel was        stained for total protein with Coomassie Blue and dried on        cellulose film. The proteins from the second gel were        electrophoretically transferred to nitrocellulose membrane as        described by Towbin et al. (1979). Following transfer, the        membrane was blocked with phosphate-buffered saline (pH 7.2) and        0.1% Tween-20 (PBS-Tween) with 2% BSA. The blot was probed with        polyclonal rabbit anti-AexT at a 1:500 dilution in PBS-Tween+1%        BSA for 2 h and washed with PBS-Tween. Alkaline        phosphatase-conjugated polyclonal goat anti-rabbit antibody        (Caltag) was applied to the membrane at a 1:2500 dilution in        TBBT for 2 h, washed in TBBT. The transblot was then developed        using NBT and BCIP as a colour reagent system at pH 9.4.

A. salmonicida Immunity: Fish (rainbow trout Oncorhynchus mykiss and/orAtlantic salmon Salmo salar) that have been determined to be pathogenfree and are at least 15 g in size are held for at least one-week prevaccination for acclimation purposes. During the acclimation period thefish are offered 1% body weight in salmonid fish food every day, howeverthey are denied food 24 hours pre and post-vaccination.

At least 50 fish are vaccinated 0.2 cc of vaccine via intra-peritoneal(IP) Injection, at the same time a group of at least 50 fish from thesame stock are mock vaccinated with 0.2 cc of saline. Vaccinated fishare then held for a period of at least 350-degree days to allow specificimmune response generation in an acclamation tank with a continuous flowof water at a temperature of 12-13° C. The fish are offered 1% bodyweight in salmonid fish food daily until 24 hours pre-challenge andpost-challenge.

After at least 350-degree days post vaccination 50 fish are challengedby immersion in, or IP injection with, a pre-determined concentration ofvirulent Aeromonas salmonicida. The dosage depends on the source of thefish and the water temperature (this is determined empiricallyimmediately prior to challenge of test fish). The identical procedure isperformed with the mock-vaccinated control fish. The fish are observeddaily for mortality for 21 days post challenge and the cause ofmortality assessed and examined to ensure that mortality is attributedto the challenge organism. After 24 hours post-challenge the fish areagain offered 1% body weight in salmonid fish feed daily. Tanks aremaintained with a continuous flow of water at a temperature of 12-13° C.For a challenge series to be considered satisfactory; all challengegroups must meet the following criteria:

-   -   1. At least 70% of the non-immunized controls must die within 21        days of challenge.    -   2. A relative percent survival (RPS) of no less than 25% must be        achieved for the challenge disease before a vaccine is        considered even partially efficacious for this disease.        RPS=[1−(% mortality vaccinates/% mortality controls)]×100

Developed from: The Rules Governing Medicinal Products in the EuropeanUnion, Volume VII, Guidelines for the testing of veterinary medicinalproducts, 1994. Specific Requirements for the Production and Control ofLive and Inactivated Vaccines Intended for Fish. Section 3.2. Potency.

Data:

-   See Excel file    Western Immunoblot:    Gel Contents:

Lane 1: Bacterin 2 cell pellet; Lane 2: Bacterin 2 culture supernatant;Lane 3: Bacterin 3 cell pellet; Lane 4: Bacterin 3 culture supernatant;Lane 5: Bacterin 1 cell pellet; Lane 6: Bacterin 1 culture supernatant.

Conclusions:

Vaccines for furunculosis (the disease caused by A. salmonicida) aremore efficacious if AexT is induced using specific culture conditions.The presence of AexT in a bacterin can be determined by WesternImmunoblotting and developing with anti-AexT antibodies.

Mortality data

A. sal Vaccinate Challenge 50 RBT of approx. 12 g were vaccinated with 2vaccine candidates. Saline controls were additionally vaccinated. 350degree days post-vaccination, all groups were challenged with A.salmonicida Challenge Culture Information OD650 nm = 0.201 = 1.0E+08cfu/mL Used 0.1 mL of 3.3E+05 cfu/mL (washed cells) per fish A. sal:washed in 0.85% saline, final titre of 3.3E+05 cfu/mL

Number of RBT Remaining TANKS C2 B1 B2 C1 Bacterin 1 Bacterin 2 +Bacterin 3 + Date Days Saline no AexT AexT AexT 26-Apr-01 0 50 50 45 5027-Apr-01 1 50 50 45 50 28-Apr-01 2 50 50 45 50 29-Apr-01 3 28 48 45 4930-Apr-01 4 9 36 45 46 01-May-01 5 8 31 45 45 02-May-01 6 4 26 44 4403-May-01 7 4 26 43 43 04-May-01 8 4 26 42 42 05-May-01 9 4 21 41 4106-May-01 10 4 21 41 41 07-May-01 11 3 21 41 41 08-May-01 12 2 21 41 4109-May-01 13 2 21 41 41 10-May-01 14 2 21 41 41 11-May-01 15 2 21 41 4112-May-01 16 2 21 41 41 13-May-01 17 2 21 41 41 14-May-01 18 2 21 41 4115-May-01 19 2 21 41 41 16-May-01 20 2 21 41 41 17-May-01 21 2 21 41 41Gross % survival 4.00 46.00 91.11 82.00 Gross % mortality 96.00 54.008.89 18.00 Gross RPS 0 44 91 81

1. An isolated protein comprising SEQ ID NO:2.
 2. An isolatedpolypeptide comprising at least one epitope or epitopic region of AexT.3. The polypeptide of claim 2, wherein the length of the polypeptide isshorter than the full length of AexT.
 4. An isolated nucleic acidfragment encoding a protein having an amino acid sequence as given inSEQ ID NO: 2, encompassing amino acid substitutions, additions anddeletions that do not alter one of the immunogenicity and the functionof said protein.
 5. An isolated nucleic acid fragment comprising SEQ IDNO:1.
 6. An isolated nucleic acid fragment comprising the contiguoussequence of SEQ ID No:1, or the full length complement thereof.
 7. Thenucleic acid fragment of one of claims 4-6 operatively linked to apromoter.
 8. The nucleic acid fragment of claim 7, wherein said promotercomprises the contiguous sequence of SEQ ID NO:3.
 9. An immunogenic,immunological or vaccine composition comprising a protein or polypeptideas claimed in any one of claims 1-3.
 10. A method for reducing thesusceptibility of a poikilothermic fish to infection by a virulentstrain of A. salmonicida comprising the administration, eitherintraperitoneally, intramuscularly, intradermally, intracellularly, byspraying, by immersion, or orally, or by any combination of routes, tosaid fish an immunogenic amount of at least one epitope or epitopicregion of AexT, its variants, or antigenic peptides derived orsynthesized thereof, with or without an adjuvant.
 11. The use of AexT ina vaccine for reducing the susceptibility of fish to infection by avirulent strain of A. salmonicida.
 12. A vaccine for reducing thesusceptibility of fish to infection by a virulent strain of A.salmonicida comprising AexT.
 13. An immunogenic, immunological orvaccine composition comprising a nucleic acid fragment as claimed in anyone of claims 4-8.
 14. A method for the inducement of AexT production inA. salmonicida comprising the growth of A. salmonicida on Ca²⁺ or othercation depleted medium.
 15. A bacterin vaccine for the immunization offish comprising killed or inactivated A. salmonicida cells in whichexpression of AexT had been induced prior to the inactivation, or inwhich A. salmonicida has been manipulated (using recombinant or othermeans) to constitutively express AexT prior to inactivation.
 16. Amethod for diagnosing the presence of A. salmonicida or other bacteriaexpressing AexT or homologues thereof comprising the detection of AexTprotein following the steps outlined in the Examples.
 17. An isolatednucleic acid fragment comprising SEQ ID NO:
 3. 18. Use of the nucleicacid fragment of SEQ ID NO: 3 in the regulation of expression ofproteins in recombinant expression systems.
 19. An isolated proteincomprising SEQ ID NO:
 4. 20. An isolated nucleic acid fragmentcomprising SEQ ID NO:
 5. 21. A method for reducing the susceptibility ofa poikilothermic fish to infection by a virulent strain of A.salmonicida comprising the administration, either intraperitoneally,intramuscularly, intradermally, intracellularly, by spraying, byimmersion, or orally, or by any combination of routes, to said fish animmunogenic amount of a pharmaceutical composition comprising thenucleic acid fragment as claimed in any one of claims 4-8, or fragments,synthetic oligonucleotides, or DNA sequence homologues thereof.
 22. Animmunological method for the detection of humoral antibody to proteinAexT or to A. salmonicida in sera of poikilothermic fish comprising theuse of the AexT protein, a fragment or synthesized peptide thereof, orAexT and a fusion partner, to adsorb or bind to fish immunoglobulin fromfish sera.
 23. Use of AexT or the DNA encoding the gene for AexT as wellas those DNA regions flanking that gene of A. salmonicida in themanufacture of a diagnostic agent.